Polarization effects in optically bound particle arrays
Department of Chemistry, Durham University, Durham, England, United KingdomOptics Express (Impact Factor: 3.49). 11/2006; 14(21):10079-88. DOI: 10.1364/OE.14.010079
Sub-micron polystyrene spheres spontaneously assemble into twodimensional arrays in the evanescent field of counterpropagating laser beams at the silica-water interface. The symmetry and dynamics of these arrays depends on the particle size and the polarization of the two laser beams. Here we describe the polarization effects for particles with diameters of 390-520 nm, which are small enough to form regular 2-D arrays yet large enough to be readily observed with an optical microscope. We report the observation of rectangular arrays, three different types of hexagonal arrays and a defective array in which every third row is missing. The structure of the arrays is determined by both optical trapping and optical binding. Optical binding can overwhelm optical trapping and give rise to an array that is incommensurate with the interference fringes formed by two laser beams of the same polarization.
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- "Optical binding force linking two micropaticles or nanoparticles (NPs) of a dimer to maintain their stable equilibrium positions has attracted great attention           . "
ABSTRACT: This study theoretically investigates plasmon-mediated optical binding forces, which are exerted on metal homo or heterodimers, induced by the normal illumination of a linearly polarized plane wave or Gaussian beam. Using the multiple multipole method, we analyzed the optical force in terms of Maxwell's stress tensor for various interparticle distance at some specific wavelengths. Numerical results show that for a given wavelength there are several stable equilibrium distances between two nanoparticles (NPs) of a homodimer, which are slightly shorter than some integer multiples of the wavelength in medium, such that metal dimer acts as bonded together. At these specific interparticle distances, the optical force between dimer is null and serves a restoring force, which is repulsive and attractive, respectively, as the two NPs are moving closer to and away from each other. The spring constant of the restoring force at the first stable equilibrium is always the largest, indicating that the first stable equilibrium distance is the most stable one. Moreover, the central line (orientation) of a dimer tends to be perpendicular to the polarization of light. For the cases of heterodimers, the phenomenon of stable equilibrium interparticle distance still exists, except there is an extra net photophoretic force drifting the heterodimer as one. Moreover, gradient force provided by a Gaussian beam may reduce the stability of these equilibriums, so larger NPs are preferred to stabilize a dimer under illumination of Gaussian beam. The finding may pave the way for using optical manipulation on the gold or silver colloidal self-assembly.
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- "Similarly, a microparticle exposed to an evanescent optical field experiences a radiative force, as was first demonstrated by the driving of water-suspended particles above the surface of a prism by a single laser beam, weakly focused and undergoing total internal reflection on the prism-to-water interface from below . Later experiments used two counter-propagating laser beams which lead to stable trapping   and the formation of a variety of selfassembled particle structures dependent on the polarization state of the laser beams  or the number of bound particles , and enhanced the optical forces by enclosing the experiment in a resonant optical cavity . An alternative scheme for generating an evanescent optical field suitable for trapping is to use an optical fiber that is tapered to a diameter of around 1 mm or smaller. "
ABSTRACT: We investigate the manipulation of microscopic and nanoscopic particles using the evanescent optical field surrounding an optical fiber that is tapered to a micron-scale diameter, and propose that this scheme could be used to discriminate between, and thereby sort, metallic nanoparticles. First we show experimentally the concept of the transport of micron-sized spheres along a tapered fiber and measure the particle velocity. Having demonstrated the principle we then consider theoretically the application to the optical trapping and guiding of metallic nanoparticles, where the presence of a plasmon resonance is used to enhance optical forces. We show that the dynamics of the nanoparticles trapped by the evanescent field can be controlled by the state of polarization of the fiber mode, and by using more than one wavelength differently detuned from the nanoparticle plasmon resonance. Such a scheme could potentially be used for selectively trapping and transporting nano- or microscopic material from a polydisperse suspension.
- "Such phenomena have increasingly been advocated as a tool for the optical manipulation and configuration of particles, and optically induced arrays have been observed in numerous experimental studies.   In this connection, the related term optical matter has also been coined, highlighting possibilities for a significant degree of interplay between optically induced inter-particle forces and other interactions such as chemical bonding and dispersion forces. Light-mediated inter-particle interactions undoubtedly offer the potential to organize large numbers of microparticles using optical force alone. "
Article: Mechanisms for optical binding[Show abstract] [Hide abstract]
ABSTRACT: The phenomenon of optical binding is now experimentally very well established. With a recognition of the facility to collect and organize particles held in an optical trap, the related term 'optical matter' has also been gaining currency, highlighting possibilities for a significant interplay between optically induced inter-particle forces and other interactions such as chemical bonding and dispersion forces. Optical binding itself has a variety of interpretations. With some of these explanations being more prominent than others, and their applicability to some extent depending on the nature of the particles involved, a listing of these has to include the following: collective scattering, laser-dressed Casimir forces, virtual photon coupling, optically induced dipole resonance, and plasmon resonance coupling. It is the purpose of this paper to review and to establish the extent of fundamental linkages between these theoretical descriptions, recognizing the value that each has in relating the phenomenon of optical binding to the broader context of other, closely related physical measurements.